1. Field of the Invention
This invention is directed for use in the field of lightning arrestors or protectors for protecting electrical signal lines against voltage surges which may occur on the line and which could be harmful to equipment or personnel.
2. Description of the Prior Art
An article titled "New Gas Tube Protector Technique for Circuits" by C. Bruce Barksdale appearing in TELEPHONY on Oct. 17, 1970, describes in some detail the more commonly used means of protecting electrical lines against induced lightning surges. Briefly, this article described the use of a gas-filled discharge tube containing three electrodes with one of the electrodes grounded and the other two electrodes connected to signal lines, such as a two-wire telephone line. Types of suitable gas-filled discharge tubes and their manner of construction and operation are described in U.S. Pat. Nos. 3,522,570 and 3,535,779 by Wanaselja. A high magnitude pulse, such as that induced in a telephone line by nearby lightning, if of a magnitude that equals the ignition or firing threshold of a gas tube, will cause the tube to conduct thereby providing a path to ground for both lines thus preventing this high surge from reaching the equipment being fed by the signal lines, such as a telephone receiver and transmitter. Both lines are grounded through the tube because all of the electrodes are contained in a single cavity and once the gas in the cavity ignites it brings all of the electrodes to the same low potential level. A major drawback of the multiple-electrode gas-filled discharge tube is its relatively high igniting potential which is ordinarily in the range of at least 200 volts and more often is in the range of 300 to 500 volts. While this range may be suitable for protection against harm to heavy equipment and may even possibly protect against harm to life, voltage surges of a lesser magnitude which are not arrested can still harm other present day equipment, especially equipment containing low voltage devices such as transistors and other semiconductor devices. Also, there is still the potential danger of injury to personnel from voltage surges which have a magnitude less than the ignition or firing potential levels.
U.S. Pat. No. 3,816,802 issued to the instant applicants describes gas-filled discharge tubes combined with step-up transformers for protecting signal lines, but the invention described therein has been found to be inoperable. The instant invention is a modification of this earlier circuit.